The fatigue and damage characteristics of frozen soil under cyclic loading are highly dependent on the three-dimensional (3D) stress state, due to the anisotropic properties of the ground. Measuring and researching the deformation behavior and fatigue failure characteristics of frozen soil under complex 3D cyclic stress states are significant for the stability assessment of frozen soil when it is subjected to earthquakes and vehicular traffic. In this paper, a hollow cylindrical apparatus was used to simulate a cyclic stress state with constant values of principal stress direction angle (alpha), coefficient of intermediate principal stress(b), and amplitude of the first principal stress under -6degree celsius conditions. The influences of 3D stress parameters (alpha and b) on the deformation behavior, damage evolution, and fatigue failure characteristics of frozen silty clay were systematically investigated. The results indicated that the deformation of the samples was dominated by axial strain, when alpha < 15 degrees and b = 0. Furthermore, as the value b increased, both the accumulated axial strain and accumulated torsional shear strain exhibited a decreasing-then-increasing trend. When 30 degrees <=alpha <= 60 degrees, the deformation feature is primarily dominated by torsional shear direction. With the increase of the value b, the accumulated torsional shear strain increased rapidly, while the axial strain gradually decreases, and then in turn to compressive elongation deformation. The increase of 3D stress parameters leads to a decrease in accumulated torsional shear strain, absolute value of accumulated axial strain, number of cycles, and accumulated torsional shear dissipated energy density at the failure of frozen soil. This indicated that under cyclic stress conditions, the increase of 3D stress characteristic parameters accelerates the damage evolution and fatigue failure process of frozen soil samples. Essentially, the increase of 3D stress parameters accelerates the damage of soil particle and ice lens structures in horizontally layered and the growth of micro-crack of frozen soil, thereby reducing the transverse shear resistance of frozen soil samples.

Wildfires considerably disturb the structure and forest ecosystem functioning. The disturbances estimation as well as the extent of damage to the soil and vegetation soon after the fire is crucial information for planning of restoration efforts. Because of the financial resources needed for field work and the involvement of experts, remote aerospace methods and data are extensively employed in monitoring ecological research. The aim of this paper is to assess postfire forest disturbances and initial regrowth processes using the tasseled cap derived Direction Angle (DA). DA is an index introduced by the authors in previous research - the angle between the Greenness component from the TCT (tasseled cap transformation) and VIC (Vector of Instantaneous Condition). The proposed method is based on linear orthogonal transformation of multispectral satellite images and is characterized with higher accuracy compared to standard methodologies using vegetation indices. The higher accuracy of the methodology is based on the linear orthogonal transformation of multispectral satellite images (TCT), which increases the degree of identification of the three main components changing during fire - soil, vegetation, and moisture/water. The methodology proposed in this paper is characterized by high accuracy in assessing the recovery of undergrowth, that is difficult to differentiate using standard monitoring methodologies based on vegetation indices. The DA raster images show the direction of change of the green tasseled cap component (TCG) relative to the VIC, which allows to estimate the degree of recovery of the vegetation component for different moments of the study period. The variations observed in DA values illustrate the pattern of the green component at various points during the investigation period, enabling the assessment of disturbances and the monitoring of regrowth processes. The test area is located in the Middle Rhodopes, near the village of Hvoyna (Smolyan region), Bulgaria, where on 28/08/2023 a wildfire broke out. 1,500 decares have been burnt by the fire, including deciduous and coniferous forest. The wildfire affected 100-130 years old black pine (Pinus nigra) forests.